This invention relates to a gib usable with various types of machine tools and more particularly to an improved tapered gib which is of low friction and may be run dry and which will accommodate various ways without critical match fitting procedures used with metal gibs.
Most machine tools include a table which is movable in an x-y direction or in three coordinates, i.e. an x-y-z direction. The table is normally mounted on a saddle which includes a lead screw or ball screw device to effect movement of the table in one coordinate direction. In some machine tools, the table is held in movable alignment with the saddle by a tapered gib received in a way, the gib being used to form an adjustable table tightening device for proper controlled precision movement of the table.
Normally, tapered gibs are made of cast iron and run in contact with a steel table, a lubricant being used between the relatively movable parts. Such a gib is normally fitted to the way such that there is a proper fit over the entire range of table movement.
As will be appreciated, such a procedure is time consuming and requires considerable care in order to assure that the sliding frictional relationship is essentially of the same magnitude over the entire range of table movement. In other words, high spots which could conceivably cause a binding action must be removed after their location has been ascertained. It will be apparent, therefore, that fitting a cast iron gib to a way requires considerable time and critical measuring and machining operations.
In the normal use of a machine tool including a tapered gib, the predominant portion of the machining is done in a midrange of table movement. Thus, by virtue of the sliding friction fit between the gib and the way, there is wear of both the gib and the way, which wear is most concentrated in that portion of the table in which the predominant amount of machining is done in the normal use of the equipment. As the gib and/or the table wear, the gib is moved axially in order to compensate for the wear thereby maintaining the table in proper alignment. That is, wear of the gib and/or the way creates "play" which is eliminated by moving the tapered gib axially in the way. The effect of such an adjustment is to increase the friction between the gib and the way on those portions of the table other than which most machining operations are made, for example, the ends of the table. Thus, as the table is moved from the center position to one or the other of the ends, the friction gradually increases with the result that the rate of movement is slowed, or requiring a larger motor in order to effect such movement. Where it is desirable to move the table at a rapid rate, for example, in connection with numerically controlled equipment, it is important that the frictional engagement between the tapered gib and the way be the same throughout the entire length of table movement.
It will also be apparent that replacement of the gib is as involved a procedure as the original fitting of the gib. The principal difficulty, however, is the nonuniform wear which occurs in the way of the table as a result of the normal use of the machine tool since the dominant portion of the machining operations generally take place in the central portion of the table.
The position of the gib axially in the way is adjustable and set by a locking device which secures the gib in position in the way. It will be appreciated, therefore, that nonuniform movement of the table is easily detected by an experienced machine tool operator, as is play which results from a worn gib or a combination of worn gib and worn way.
Accordingly, it becomes quite apparent that the disadvantages of the use of metal tapered gibs create serious practical problems in machine tool construction, and/or repair, which problems are easily detected by the machine tool operators. It is not uncommon for an experienced machine tool operator to characterize a particular machine tool as a "bad" machine in the context that the table is difficult to move in certain directions or inaccurate in its movement, or both. Frequently the source of the problem is the improper adjustment of the gib, or a worn gib which results in play of even a small amount which affects the position of the table and therefore the accuracy of the machining operation. Such inaccuracies due to the gib structure, alignment, or position are particularly critical in the case of numerically controlled machines. For example, the fact that a table may be worn more in the midsection than at the ends results in a different backlash condition as the table travels in the midsection. In numerically controlled equipment, this requires a complex differential in settings to accommodate the different backlash conditions. In some instances, the adjustment of the gib is off to such an extent that numerically controlled equipment cannot be used because of the inability to reach precise accurate positions as controlled by the numerically controlled equipment.
Accordingly, the provision of a gib of a tapered type which provides a uniform low friction sliding surface for control of the table is definitely advantageous, particularly if the gib is of a type which need not be fitted to the particular way as has been the case with tapered metal gibs of the prior art. For example, a gib which is of sufficient strength and wear characteristics as well as of a conformable material in the sense that it will accommodate the small variations in the way overcomes the problem of critical matching of the gibs with the ways during the course of building and/or repair of machine tools.
It is known in the prior art to utilize low friction material as a coating or a layer on the surface of the gib which is in sliding contact with the way. Such an arrangement has the disadvantage of wearing differentially, that is, wearing at the portions which are in the maximum frictional engagement with the relatively movable way. Also, such an arrangement requires special machining of the metal gib to remove sufficient metal to accommodate the added thickness of the low friction material adhered or otherwise secured to the face of the gib. While the use of such a layer or material is an improvement over an all metal gib, the fact that the material is essentially a flat sheet fixed between the metal gib and the metal way results in differential wear of the material and ultimately leads to play.
It is also advantageous to provide a gib which does not bring about wear of the way, and which does not bring about nonuniform wear of that portion of the way located in the area in which the major portion of the machining operations are accomplished.